Two-dimensional surface relief grating (2D-SRG) on azo-dye doped polymer film (DDPF) was fabricated using the two-step of holographic writing technique. The groove structure of the 2D-SRG was dependent on the rotational angle of the two-step of writing, and the depth of the groove could be enhanced about 2~3 times by using nematic liquid crystal as the interface. The surface modulation of groove on DDPF with or without the interface of nematic liquid crystal depending on the polarization of the writing beams is also discussed. As the rotational angle between the two steps of writing was increased, the depth of the groove was gradually decreased for the writing beams with the S-polarization in the second step of writing, while increasing for the writing beams with the P-polarization.
© 2014 Optical Society of America
Azo polymer materials have been received much attention in recent years because of their potential applications in optical elements , optical data storage , optical switching devices [3,4] and photoalignment [5–7]. Todorov et al. demonstrated that the birefringence phase grating could be fabricated on azobenzene polymer film by an interference pattern of a pair of linearly polarized lights . The trans-cis photoisomerization of the azobenzene group introduces the alignment of the molecular long axis of nematic liquid crystal along the direction perpendicular to the polarization of the incident beams . However, the exposure of azo polymer film with an interference pattern of coherent light could further induce the surface modification on thin film. Kim et al. was the first group to report the surface relief grating on polymer films . The reorientation of the azobenzene group induces the large scale molecular motion, and results in the mass transport of the irradiated azopolymer thin film. Such SRG recorded by two orthogonal linearly polarized lights was also observed in side-chain azobenzene polyesters . If the grating period and depth of SRG were narrow and deep enough, it possesses the potential application as the alignment layer in liquid crystal cell. The formation of SRG could produce a preferred orientation to the overlying liquid crystal. The alignment of nematic liquid crystal along the groove is due to the minimization of the elastic strain energy .
Recently, the multiplexing technique had been used in several materials, such as azo-dye doped liquid crystal [13,14], photopolymer [15–17], ptotoresist , and azo polymer film [10,19–23]. This technique is mainly used for investigating the more complicated formation of grating. By using the technique of multiple holographic recording, the features of the high optical capacity and recording quality could be achieved. The properties of SRG recorded with polarization multiplexing in the azo polymer film were investigated by Ilieva et al. . The optimal conditions for multiplexing were found with the scalar hologram for the first recording and the polarization hologram for the second recording in two writing beams with linear polarizations. Owing to the highly symmetry same as the multiple holographic recording, the 2D-SRG could be utilized as a template for fabricating the well-ordered TiO2 nanostructures . Moreover, the 2D-SRG fabricated on the azo polymer film at a fiber surface end had been employed as a beam splitter, beam deflector, and diffractive lens .
In the previous studies, it took longer (usually many seconds or more) to fabricate the SRG by using a continuous wave (CW) laser [7–11,19–26]. Hill et al. demonstrated the holographic recording on the side-chain polymer film by a single pulsed laser . The rapidly optical recording could induce the fast photoisomerization of azobenzene molecule. In this study, we have demonstrated the feasibility of increasing the surface modulation of grating in contact with nematic liquid crystal by using the fast holographic recording operated at a single pulse, and the deeper groove could be developed under the condition of the polarization of writing beams parallel to the molecular director of NLC [28,29]. In order to fast fabricate the high quality of 2D-SRG and find out more details about the change of surface modulation, we report the fabrication of 2D grating on DDPF using a two-step holographic recording in multiplexing technique . The morphology of 2D grating on DDPF with or without the interface of nematic liquid crystal will be discussed. The dependence of the surface modulation on the rotational angle between the two writing steps will also be investigated.
A polymer solution was prepared by dissolving azo dye (DR1, Aldrich), 4% of weight concentration, and polymethyl methacrylate (PMMA, Aldrich), 96% of weight concentration, into a toluene solvent. The prepared polymer solution was then spin-coated onto a glass substrate. The thickness of the resultant film was measured to be 4 μm. The sample cell was assembled with a pair of glass substrates. One substrate was coated with DDPF, while the other substrate was coated with a polyimide (PI) layer, which was rubbed for the homogeneous alignment. The cell gap was controlled by a pair of teflon spacers with the thickness of 12 μm. In this experiment, two kinds of samples were used to fabricate surface relief grating on azo-dye doped polymer film. The sample of DDPF was prepared without the interface of nematic liquid crystal, while the sample of NLC/DDPF was prepared with the injection of nematic liquid crystal (5CB, ne = 1.7063, no = 1.5309 at 633 nm, Merck) into the sample cell. The experimental setup for the fabrication of 2D-SRG on the DDPF is shown in Fig. 1.The SRG was produced by two equal intensities of coherent writing beams from the Nd:YAG pulsed laser operated at a wavelength of 532 nm with a pulse width of 6 ns and an energy density of 0.2J/cm2 for each writing beam. The interference pattern was produced by the writing beams with the S-polarization (parallel to the Z-direction). The grating pitch Λ corresponding to the interfringe spacing of the interference pattern is given by
where λ is the wavelength of the Nd:YAG pulsed laser and θ is the incidence angle of the writing beams onto the sample. The angle between two writing beams is 6° and the periodicity of the grating is about 5μm according to Eq. (1).
To fabricate the two dimensional structure of surface relief grating, the sample was fixed on a rotational stage, which could be rotated around the Y-axis. For the NLC/DDPF, the first surface relief grating was recorded at a fixed molecular direction parallel to the polarization of writing beams. Under this arrangement of geometrical configuration, the optimal quality of the first grating could be achieved and the investigation of the dependence of surface modulation on rotational angle could be simplified for S- and P-polarization of writing beams. Therefore, the second surface relief grating was recorded again after the stage was rotated by an angle of φ. Both the recording of the first and second surface relief grating were only operated at a single pulse. The two-dimensional structure of the surface relief grating was developed by superimposing one grating onto another [18,22]. The unpolarized He-Ne laser (633nm) was introduced directly to the sample to serve as the probe beam. The first-order diffraction intensity of sample was monitored to diagnose the quality of SRG. The higher diffraction intensity implies the larger surface modulation of SRG. After the completion of recording the 2D-SRG, the sample cell was detached carefully; and nematic liquid crystal molecules sticking on the surface of SRG were removed with the solvent of n-Hexane. Afterward, the Atomic Force Microscopy (AFM) was employed to measure the depth of structure of the 2D-SRG.
The two-dimensional surface relief grating was fabricated by two steps of grating formation. The structure of 2D-SRG was determined by the intensity and the polarization of writing beams, and the rotational angle between two steps of writing. In order to understand how the rotational angle affect the contour of surface relief grating, the AFM images of 2D-SRG on DDPF sample were recorded under both writing beams with S-polarization at the rotational angles φ of 30°, 45°, 60°, and 90°, as shown in Figs. 2(a)–2(d), respectively. The contours of 2D-SRG shown in Figs. 2(a)–2(c) look like the network structure, which were formed by two gratings overlapping with angles of 30°, 45°, and 60° relative to the polarization of writing beams, respectively. The contour of 2D-SRG shown in Fig. 2(d) looks like the egg-crate structure, which was formed by two gratings overlapping perpendicularly.
In order to investigate the orientational dependence of the surface modulation in the second writing, the surface modulation of the first grating was fixed at about 425nm. The depth of surface modulations of the SRG in the second writing were measured about 368, 378, 377, and 380nm at the rotational angles of 30°, 45°, 60°, and 90°, respectively. The depth of surface modulation of the SRG in the second writing was less than that of the first writing. The result is due to the extent of photoisomerization of azo dye molecules from the trans to the cis state. The azo dye molecules were excited from the trans to the cis state, inducing the surface modulation. In the first writing, some extent of azo dye molecules had been excited from the trans to the cis state, and the number of residual molecules in trans state were reduced. Therefore, the depth of the surface modulation was shallower due to less photoisomerization in the second writing.
The AFM images of the 2D-SRG on NLC/DDPF sample were recorded under both writing beams with S-polarization at the various rotational angles φ of 30°, 45°, 60°, and 90°, as shown in Figs. 3(a)–3(d), respectively.
With the interface of nematic liquid crystal, the surfaces of the relief structures of NLC/DDPF were smoother compared with that of the DDPF sample. The depth of the SRG was fixed at about 1.35μm in the first writing with the same energy density as the DDPF sample, which was around three times deeper than that of the DDPF. The result of enhancement was confirmed in the previous work . In the second writing, the depth of the surface modulations at the various rotational angles φ of 30°, 45°, 60°, and 90° were about 1.13, 1.07, 0.95, and 0.75μm, respectively. The result shows that the depth of the SRG in the first writing is deeper than that of the second writing, which is the same as for DDPF. The holographic recording of 2D-SRG on DDPF sample and NLC/DDPF sample under both writing beams with P-polarization (parallel to the X-direction in Fig. 1) were also investigated. The depth of SRG was about 650nm for DDPF sample and 1.76μm for NLC/DDPF sample in the first writing. In addition, the depth of the surface modulations in the second writing at the various rotational angles φ of 30°, 45°, 60°, and 90° were about 554, 559, 554, and 568nm for DDPF sample and 1.24, 1.31, 1.37, and 1.53μm for NLC/DDPF sample, respectively.
The dependence of the depth of SRG in the second writing on the rotational angle for DDPF sample and NLC/DDPF sample is shown in Fig. 4.For the case of DDPF sample, the surface modulation of SRG in the second writing fluctuated between 365nm and 380nm under the writing with S-polarization, and between 555nm and 570nm under the writing with P-polarization when the rotational angle was increased. It can therefore be concluded that the depth of the surface modulation in the second writing were independent of the rotational angle. The fluctuation of surface modulation is because of the formation process of SRG via the free expansion without any influence of nematic liquid crystal. However, the photoexpansion effect was induced in the azo dye doped polymer film under the exposure of the second harmonic of Nd:YAG pulsed laser.
For the case of NLC/DDPF sample, the depth of SRG in the second writing with S-polarization was gradually decreased as the rotational angle was increased. On the contrary, the depth of SRG in the second writing with P-polarization was gradually increased as the rotational angle was increased. The depth of the second relief grating for NLC/DDPF sample depends on the polarization of the writing beams and the rotational angle between the two steps of writing. In order to investigate the dependence of surface modulation on polarization and rotational angle, two geometric configurations between the polarization of the writing beams and the molecular director of nematic liquid crystal were defined. The polarization of writing beams was parallel to the molecular director of nematic liquid crystal in the parallel configuration, while perpendicular to the molecular director of nematic liquid crystal in the perpendicular configuration. It had been reported that the strong interaction force between azo dye molecules and nematic liquid crystal led to the deeper depth of the SRG in the parallel configuration , while the weak interaction force led to the shallower depth of the SRG in the perpendicular configurations. The formation process of 2D-SRG on NLC/DDPF under S- and P-polarization of writing beams was schematically illustrated in Fig. 5. In the first writing with S-polarization, the first grating was formed in a parallel geometrical configuration, as shown in Fig. 5(a). The direction of the first grating was parallel to the Z-axis and the molecular director of nematic liquid crystal on the SRG was along the original direction (Z-axis). The configuration of the polarization of writing beams and the molecular director of nematic liquid crystal was gradually turned from parallel to perpendicular when the rotational angle was increased in the second writing, as shown in Fig. 5(b). Therefore, the depth of SRG was gradually decreased as the rotational angle was increased.
In the first writing with P-polarization, the first grating was formed in a parallel geometrical configuration, as shown in Fig. 5(c). The direction of the first grating was also parallel to the Z-axis and perpendicular to the orientation of nematic liquid crystal. According to the groove theory , the nematic liquid crystal was reoriented along the direction of grating on the DDPF and along the rubbing direction on the PI film due to the surface anchoring energy, as shown in Fig. 5(d). Therefore, the conformation of the sample cell was the twisted nematic (TN) liquid crystal. As the rotational angle was increased, the geometrical configuration was gradually changed from perpendicular to parallel, which led to a gradual increase in the depth of SRG in the second writing. Therefore, the optimal condition for producing a high quality of 2D-SRG is that the NLC/DDPF sample was rotated with 90° in the second writing with P-polarization.
In conclusion, the dependence of surface modulation of SRG in DDPF and NLC/DDPF on the rotational angle in the second writing was reported. For DDPF sample, the surface modulation in the second writing smaller than that of the first writing is due to fewer dye molecules under photoisomerization from the trans to the cis state. The result shows that the surface modulation in the second writing was independent of the rotational angle. For NLC/DDPF sample, the depth of the SRG in the second writing was gradually decreased for the writing beams with S-polarization, and increased for the writing beams with P-polarization as the rotational angle was increased. The deeper depth of the surface modulation was due to the strong interaction force between azo dye molecules and nematic liquid crystal when the polarization of writing beams was parallel to the molecular director of nematic liquid crystal. The high quality structure of 2D-SRG based on NLC/DDPF could be utilized in various applications such as beam splitter, surface bistable alignment, lithography technique, and nanostructures.
The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract Nos. NSC 102-2112-M-110-007.
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